Inkjet printers have conventionally used as machines for carrying out printing on paper. In the meanwhile, focusing attention on the versatility and cost-effectiveness of the ink jet printers, it has been attempted to use an inkjet printer for the processes that have conventionally been done by photolithography, e.g. the formation of a miniature pattern such as a color filter of a liquid crystal display device, and the formation of a conductor pattern on a print circuit board. For this reason, it has become popular to develop a minute dot formation device that can form a highly-precise minute pattern by directly forming minute ink dots on a drawing object (e.g. a print circuit board and a color filer for liquid crystal display).
Such a minute dot formation device requires a nozzle plate having excellent discharging characteristics, e.g. stable discharge and highly precise landing of droplets.
The following will describe the structure and manufacturing method of a conventional nozzle plate. Japanese Laid-Open Patent Application No. 9-216368 (published on Aug. 19, 1997) discloses a technology to form a nozzle plate by dry etching and wet etching. FIGS. 19(a) and 19(b) illustrate the nozzle plate (hereinafter, conventional nozzle plate) of the above-described Patent Document 1.
The conventional nozzle plate is made up of an SOI (Silicon on Insulator) substrate 21. As shown in FIGS. 19(a) and 19(b), the SOI substrate 21 is arranged such that an SiO2 layer 26 which is an etching stop layer covers the entirety of a supporting silicon layer 25, and a silicon layer 24 which is an active layer is further provided on the SiO2 layer 26. The silicon layer 24 has an orifice 22, the silicon layer 25 has a tapered portion 23, and the orifice 22 is connected with the tapered portion 23.
The conventional method of manufacturing the nozzle plate (hereinafter, conventional method) is arranged as follows. First, the surface of the silicon layer 24 which is an active layer is oxidized, so that an oxidized film (not illustrated) is formed. Then a predetermined pattern is formed on the oxidized film 28. Using this pattern as a mask, dry etching is performed. At this time, the etching does not go beyond an SiO2 layer 26 that functions as an etching stop layer. As a result, the orifice 22 is formed. Subsequently, the surface of the silicon layer 25 which is a supporting layer is oxidized, so that an oxidized film (not illustrated) is formed. A predetermined pattern is formed on this oxidized film. Using the pattern as a mask, dry etching is performed in an undercut manner so that the etching is stopped at the SiO2 layer 26. As a result, the tapered portion 23 is formed. Finally, the SiO2 layer 26 and the surface oxidized film, which are between the orifice 22 and tapered portion 23, are etched away using a fluoric etchant.
The arrangement above, however, has the following problems.
Since the SiO2 layer 26 which is the etching stop layer is entirely formed between the silicon layers 24 and 25, the nozzle plate may be greatly warped by the stress due to the difference of linear expansion coefficients between silicon and SiO2. The warpage of the nozzle plate induces not only the decrease in precision of the joining between the nozzle plate and an inkjet head but also the decrease in the structural reliability of the nozzle plate.
In the conventional nozzle plate, sufficient rigidity of the silicon layers 24 and 25 is required to avoid the aforesaid warpage of the nozzle plate due to the difference of linear expansion coefficients between silicon and SiO2. Therefore, it is unavoidable to increase the thickness of the silicon layer 24 where the orifice 22 is formed and the thickness of the silicon layer 25 where the tapered portion 3 is formed (silicon layer 24 is 15 μm thick and SI layer 25 is 100 μm thick).
This increases an amount of silicon to be etched away at the time of the formation of the orifice 22 and the tapered portion 23, thereby increasing an error in the etching. That is, the precision in the formation of the nozzle (i.e. orifice 22 and tapered portion 23), through which droplets flow, is decreased. On this point, Patent Document 1 describes that the processing accuracy of the nozzle is not more than ±1 micrometer, as compared to the planned sizes. This processing accuracy is not sufficient for minute dot formation devices.
The present invention was done to solve the above-described problems, and the objective of the present invention is to create a nozzle plate that has a highly-precise first nozzle hole and is not liable to deformations such as warpage, and the objective is also to create a manufacturing method of the nozzle plate.